1. Field of the Invention
The invention relates to the discovery of compounds and their use in the selective control of blue-green algae, also referred to as cyanobacteria, (Cyanochloronta) in managed bodies of water, and deals particularly with the use of certain derivatives of 9,10-anthraquinone for such a purpose.
Geosmin and 2-methylisoborneol (MIB) are released into the pond water from producing species of cyanobacteria, and these compounds are quickly absorbed into the adipose tissue of catfish. Producers must hold catfish that are determined to be off-flavor by trained individuals at processing plants until they are deemed to be “on-flavor.” These delays in harvest can last for several days or weeks depending upon the lipid content of the catfish, water temperature, and severity and longevity of the musty off-flavor episode in the production pond. Such delays result in economic losses to the producer due to: 1) additional feed costs; 2) interference with cash flow; and 3) the potential loss of diseased fish due to disease and predation.
Use of the instant invention is for the control of 2-methylisoborneol in water for the elimination of musty off-flavor in water and catfish raised in it. This condition costs the catfish industry up to $60 million dollars annually. The active agents do not kill off green algae at dosing concentration but are focal in their effect on blue-green algae. This allows for the maintenance of a more stable group (non-bloom forming) of diverse planktonic organisms which makes problems involving oxygen depletion and the build up of ammonia resulting from die-offs of the blue-green algae more subject to controlled management.
2. Description of the Prior Art
There are numerous chemical agents that are known to either kill or inhibit blue-green algae growth but each possess aspects limiting their usefulness with catfish rearing. Several agents kill all algae species and cause the creation of negative growth conditions, such as the development of low oxygenation conditions and a rise in ammonia concentrations when used. It is also common knowledge that several compounds must be applied at rates that are toxic not only to algae but also to fish. Another problem is that many compounds tend to persist in the aqueous environment for excessive periods of time. The closest prior art of which the applicants are aware is a publication by Schrader et al. entitled “Selective Growth Inhibition of the Musty-Odor Producing Cyanobacterium Oscillatoria cf. chalybea by Natural Compounds;” Bull. Environ. Contam. Toxicol. (1998)60:651-658 in which it is disclosed that 9,10-anthraquinone has potent activity toward Oscillatoria perornata, however insolubility in water precluded its usage to control Oscillatoria perornata in catfish ponds.
One of the management practices used by producers to prevent musty off-flavor episodes involves the application of algicides to fish ponds in order to kill or help prevent the growth of undesirable cyanobacteria. Copper sulfate, chelated-copper compounds, and the herbicide diuron are currently the only compounds approved by the United States Environmental Protection Agency (USEPA) for use in food-fish production ponds as algicides. In 1999, catfish farmers were granted an emergency exemption by the USEPA and United States Food and Drug Administration to permit the use of diuron as an algicide in catfish aquaculture ponds. However renewal of the exemption must occur annually and is not assured. Unfortunately, these algicides have the following undesirable characteristics: 1) broad-spectrum toxicity towards phytoplankton can result in the death of the entire phytoplankton community and subsequent water quality deterioration leading to the death of catfish; 2) long persistence of the compounds in the environment creates concerns about environmental safety; and 3) the public's negative perception of the use of synthetic herbicides (C. S. Tucker, Off-flavor problems in aquaculture. Rev. Fish. Sci. (2000), vol. 8, pp. 45-88).
Green algae (division Chlorophyta) are the preferred type of phytoplankton over cyanobacteria in catfish production ponds for several reasons (H. W. Paerl and C. S. Tucker, Ecology of blue-green algae in aquaculture ponds. J. World Aquacult. Soc. (1995), vol. 26, pp. 109-131), including the following: 1) green algae have never been linked to off-flavor problems in farm-raises catfish; 2) green algae provide a more substantial base for aquatic food chains than cyanobacteria; 3) green algae are better oxygenators of the water than cyanobacteria; and 4) certain types of cyanobacteria can produce toxins and, in one case, have been implicated in causing the deaths of farm-raised catfish. The discovery of environmentally-safe, selective algicides that help prevent the growth of cyanobacteria responsible for causing musty off-flavor in pond-cultured catfish would greatly benefit the catfish aquaculture industry.
Previous research (K. K. Schrader, M. Q. de Regt, P. D. Tidwell, C. S. Tucker and S. O. Duke, Selective growth inhibition of the musty-odor producing cyanobacterium Oscillatoria cf. chalybea by natural compounds. Bull. Environ. Contam. Toxicol. (1998a), vol. 60, pp. 651-658) has identified several natural compounds from plants that are selectively toxic towards O. perornata. Of these compounds, 9,10-anthraquinone has a high degree of selective toxicity towards O. perornata in the laboratory and inhibits photosynthesis (K. K. Schrader, F. E. Dayan, S. N. Allen, M. Q. de Regt, C. S. Tucker, and R. N. Paul, Jr., 9,10-Anthraquinone reduces the photosynthetic efficiency of Oscillatoria perornata and modifies cellular inclusions. Int. J. Plant Sci. (2000), vol. 161(2), pp. 265-270). Pond efficacy testing of 9,10-anthraquinone dissolved in ethanol (anthraquinone is insoluble in water) using limnocorrals (K. K. Schrader, C. S. Tucker, M. Q. de Regt and S. K. Kingsbury, Evaluation of limnocorrals for studying the effects of phytotoxic compounds on plankton and water chemistry in aquaculture ponds. J. World Aquacult. Soc. (2000), vol. 31, pp. 403-415) did not effectively reduce numbers of O. perornata or reduce MIB levels compared to positive laboratory results (unpublished observations). Additional pond efficacy testing of several different formulations of 9,10-anthraquinone (e.g., incorporation with hydoxypropylmethyl-cellulose or HPMC; Tween 80 and canola oil emulsion) to maintain sufficient phytotoxic levels, determined by laboratory tests, of anthraquinone towards O. perornata in the water column also did not produce positive results (unpublished observations).
While various methodologies for the control of blue-green algae exist, there remains a need for the creation of alternate viable and cost-effective compounds for the selective control of blue-green algae without causing the creation of negative side effects for fish.